\(B\) -physics plays a central role in the global efforts to constrain the CKM unitarity triangle. The ratio of neutral \(B\) -meson mixing, e.g., is used in the unitarity triangle fits [1]}, [2]}, [3]}. Neutral \(B\) -mesons mix with their anti-particle under the exchange of two \(W\) -bosons as depicted by the box-diagrams in Fig. REF . There \(q\)  denotes a light \(d\) - or \(s\) -quark building either a \(B\) - or a \(B_s\) -meson, respectively. In the experiments, e.g., BaBar, Belle, CDF or LHCb, \(B_q\) -mixing is measured in terms of the oscillation frequencies (mass differences) \(\Delta M_q\)  and in the Standard Model (SM) this process is parameterized by [4]}
<FIGURE>\(\Delta M_q = \frac{G_F^2m^2_W}{6\pi ^2} \eta _B S_0 M_{B_q}{f_{B_q}^2B_{B_q}} \vert V_{tq}^*V_{tb}\vert ^2,\) 
